Future — Tsunami
Although many people call tsunamis "tidal waves", they are not related to tides but are rather a series of waves, or "wave trains", usually caused by earthquakes. Tsunamis have also been caused by the eruption of some coastal and island volcanoes, submarine landslides, and oceanic impacts of large meteorites — some incidents have produced a tsunami large enough to be categorised as a Mega Tsunami.
Tsunamis travel as fast as aircraft, and their destructive effects can be enormous. The destruction is caused by the amplification in the height of the waves as the waves approach the shoreline. The inflow and outflow of the water mentioned above causes huge damage to property as well as risk to life.
The evidence of the effects of large tsunamis, in terms of relocated rocks, is found widely, the most extreme example being in Hawaii where unconsolidated coral is found at 326 metres above sea level. Tsunamis can travel far around the Earth with devastating effects:
Tsunamis generally travel very fast across the ocean (typically 500km/h or more). In deep water the tsunami height might not be great but the height can increase dramatically when they reach the shoreline because the wave slows in shallow water and the energy becomes more concentrated. In addition to the inherent increase in the height of the wave from this shoaling effect, the momentum of the wave might cause it to reach a considerable height as it travels up sloping land. It is typical for multiple waves to result from one tsunami-generating event and these could be several hours apart when they reach a distant shore.
In recent years, there has been interest on the potential for mega tsunami generation from asteroid impact, large underwater slides and massive slope failures of oceanic island strato-volcanoes. Back in 1953 two geologists traveled to a remote bay in Alaska looking for oil. They gradually realised that in the past the bay had been struck by huge waves, and wondered what could have possibly caused them. Five years later, they got their answer. In 1958 there was a landslide, in which a towering cliff collapsed into the bay, creating a wave half a kilometre high, higher than any skyscraper. The true destructive potential of landslide-generated tsunami, which scientists named "Mega-tsunami", suddenly began to be appreciated. If a modest-sized landslide in Alaska could create a wave of this size, what havoc could a really huge landslide cause?
Asteroid (NEO) impacts are capable of producing tsunamis much larger than that associated with earthquakes, and may occur anywhere in the oceans. Some two-thirds of the Earth's surface is covered by the oceans so that the chances of an impact are greater there than on land. As in the case of impact on land, a "crater" is produced in the water but such craters are unstable and rapidly refill. A 30-kilometre crater penetrates through all but the deepest ocean depths. These flows create a series of deep-water waves, so-called tsunamis, which propagate outwards from the point of impact.
Tsunami from the Chicxulub impact deposited material widely and often far inland; recognition of such deposits in Haiti, Texas and Florida helped to confirm the nature and location of the event. NEOs that are small, or small fragments of larger objects that break up in the atmosphere, do not usually reach the Earth's surface. But for objects in the range of between 200 to 1,000 metres, tsunamis may be the most devastating of all the consequences of an impact, because so much of the Earth's population lives near coasts. Some studies have indicated that an impact anywhere in the Atlantic of an object 400 metres in diameter would seriously affect coasts on both sides of the ocean by tsunamis as much as 10 metres or more in height at the shore line. Read more here.
In 2001, scientists predicted that a future eruption of the unstable Cumbre Vieja volcano in La Palma (an island of the Canary Islands) could cause a super giant undersea landslide (see Wynn & Masson and Ward & Day). Later research showed that the threat was less than had originally been theorized. The next volcanic eruption is expected in the second half of the 21st century, but it is not necessarily the eruption that causes an immediate landslide. In the worst-case scenario, the western half of the island (weighing perhaps 500 billion tonnes) would catastrophically slide into the ocean. Such a landslide could cause a 100m mega-tsunami to devastate the coast of northwest Africa, with a 10-25m tsunami reaching the east coast of North America 7-8 hours later causing massive coastal devastation and the deaths of perhaps thousands of people, threatening Miami, suburbs of New York, and parts of Boston, and all coastal cities in between.
Mega Tsunamis as an indication of NEO strikes
At the southern end of Madagascar lie four enormous wedge-shaped sediment deposits, called chevrons, that are composed of material from the ocean floor. Each covers twice the area of Manhattan with sediment as deep as the Chrysler Building is high.
On close inspection, the chevron deposits contain deep ocean microfossils that are fused with a medley of metals typically formed by cosmic impacts. And all of them point in the same direction — toward the middle of the Indian Ocean where a newly discovered crater, 18 miles in diameter, lies 12,500 feet below the surface.
The explanation is obvious to some scientists. A large asteroid or comet, the kind that could kill a quarter of the world’s population, smashed into the Indian Ocean 4,800 years ago, producing a tsunami at least 600 feet high, about 13 times as big as the one that inundated Indonesia nearly two years ago. The wave carried the huge deposits of sediment to land.
Most astronomers doubt that any large comets or asteroids have crashed into the Earth in the last 10,000 years. But the self-described “band of misfits” that make up the two-year-old Holocene Impact Working Group say that astronomers simply have not known how or where to look for evidence of such impacts along the world’s shorelines and in the deep ocean.
Scientists in the working group say the evidence for such impacts during the last 10,000 years, known as the Holocene epoch, is strong enough to overturn current estimates of how often the Earth suffers a violent impact on the order of a 10-megaton explosion. Instead of once in 500,000 to one million years, as astronomers now calculate, catastrophic impacts could happen every 1,000 years.
The researchers, who formed the working group after finding one another through an international conference, are based in the United States, Australia, Russia, France and Ireland. They are established experts in geology, geophysics, geomorphology, tsunamis, tree rings, soil science and archaeology, including the structural analysis of myth. Their efforts are just getting under way, but they will present some of their work at the American Geophysical Union meeting in December in San Francisco.
In 2006 the group started using Google Earth, a free source of satellite images, to search around the globe for chevrons, which they interpret as evidence of past giant tsunamis. Scores of such sites have turned up in Australia, Africa, Europe and the United States, including the Hudson River Valley and Long Island.
Tsunamis in the Mediterranean
Tsunamis can occur in European waters due to earthquakes caused by the African Plate drifting northwards underneath the Eurasian Plate.
Ten percent of all tsunamis worldwide occur in the Mediterranean. On average, one disastrous tsunami takes place in the Mediterranean region every century. Geological research and historical records report of many powerful tsunamis that have taken the lives of thousands over the ages. Greece and southern Italy are mostly affected.
The greatest known Tsunamis in the Med.
21 May 2003: After a quake near the coast of Boumerdes, Algeria a tsunami was generated which destroyed over 100 boats on Mallorca and flooded Palmas Paseo Maritimo.
17 August 1999: A large destructive earthquake struck north-west Turkey and generated a local tsunami within the enclosed Sea of Marmara. It occurred along the Northern Anatolian Fault zone. Its epicentre was in the Gulf of Izmit. Official estimates indicated that about 17 000 people lost their lives and thousands more were injured.
9 October 1963: Tsunamis can develop not only in oceans: In Italy, near the town of Longarone, the entire northern slope of Mount Toc slid into the Vaiont dam. The water spilled over the dam and destroyed a number of villages with a wave of 140 metres. 4 000 people lost their lives.
9 July 1956: The best documented and most recent tsunamigenic earthquake in the Aegean Sea between Greece and Turkey is the one that occurred near the south-west coast of the island of Amorgos, killing 53 people, injuring 100 and destroying hundreds of houses. The waves were particularly high on the south coast of Amorgos and on the north coast of the island of Astypalaea. At these two places, the reported heights of the tsunami were 25 and 20 m, respectively.
28 December 1908: Due to an earthquake and the ensuing tsunami, the city of Messina in Italy was almost completely destroyed. More than 75 000 people were killed.
1 November 1755: The Portuguese capital of Lisbon and its inhabitants were particularly badly hit by an earthquake that occurred in the eastern Atlantic Ocean. Two thirds of the city was destroyed from resulting fires. The people seeking refuge from the flames on the banks of the Tejo River were surprised by huge flood waves produced by the earthquake. Some 60 000 people lost their lives. The waves were even observed in Ireland and on the other side of the Atlantic on the Lesser Antilles. On the coastline of the Madeira Islands the waves still had a height of 15 metres.
1672: The Cyclade islands, especially Santorini, were shaken by an earthquake The island Kos, in the east, was completely swallowed by the ensuing tsunami.
26 September 1650: A destructive earthquake was accompanied by a submarine explosion from the Colombo Volcano, which crater lies in the sea to the northeast of the island of Santorini. There was a devastating tsunami observed on the island of Ios, north of Santorini, and waves of up to 16 m were reported.
1303 AD: The quake with an estimated strength of 8 destroyed the island Rhodos and the eastern part of Crete. It caused a tsunami which reached the Egyptian coast.
365 AD: The quake of 8 to 8,5 in the year 365 caused heavy destruction on the whole of Crete. The tsunami that developed because of the quake destroyed complete coastal regions as far as Egypt and eastern Sicily. Records indicate that 50 000 people lost their lives in Alexandria.
1628 BC: The coasts of the entire eastern Mediterranean were submerged by flood waves of up to 60 metres high. The wave, caused by a volcanic eruption on Santorini, a Greek island in the Aegean Sea, and is believed to be responsible for the destruction of the Minoan culture.
The French Med. Coast
Recorded French tsunamis are catalogued here.
The map below sections the French Mediterranean coast according to the estimated level of vulnerability. This sectioning takes into account the maximum estimated sea level near the coast and the a priori possible run-up level, irrespective of the initial source of origin (earthquake or landslide). This represents an initial evaluation of the French coasts' exposure to tsunamis.